Apparatus for producing multi-component glass fiber preform

ABSTRACT

An apparatus for producing a multi-component glass fiber preform includes a multi-conduit burner 31 having five concentric conduits 31a-31e, the center three conduits (a), (b) and (c) being flush with each other at their ends, the fourth conduit d, interposed between the third (c) and outermost conduit (e) extends axially beyond the first three. The burner has a flange 33 to direct the flame onto a substrate. 
     In operation glass raw material is fed through the inner-most conduit (a) by a carrier gas. The material emitted from conduits (a -d) is mixed in a mixing area 32. Glass raw material and a nebulized dopant salt solution are emitted and burned with hydrogen gas depositing particulate glass material or soot on an adjacent substrate to produce a multi-component glass fiber preform.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of our earlier application Ser. No.189,856 filed Sept. 22, 1980, now U.S. Pat. No. 4,336,049.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for making amulti-component glass fiber preform for fabricating optical fibers foruse as transmission lines in communication systems. The term"multi-component preform" denotes a preform composed of a plurality ofcomponents.

Conventional optical fibers used as transmission lines of opticalcommunication include silica glass fibers, multi-component glass fibersand ionic crystal fibers. It is required that these optical fibers havelow loss, are inexpensive to manufacture and can employ wide bandsignals. Further, these optical fibers are required to enable easyconnection between the fibers and to have a high mechanical strength.

Conventional multi-component glass fibers comprise silica SiO₂ and adopant composed of at least one metal salt selected from the groupconsisting of alkali metal oxide such as Na₂ O, alkaline earth metaloxide such as MgO, oxide of lead such as PbO and oxide of lanthanum suchas La₂ O₃. Such multi-component glass fibers can be easily fabricatedfrom a preform since they have a relatively low melting temperature.Such multi-component glass fibers have another advantage that they havelow loss since Rayleigh scattering involved is kept to a low level.

Various methods have been proposed for making a multi-component glassfiber preform. With these methods, starting materials are firstsubjected to extremely high purification and then are mixed together.Thereafter, the so mixed materials are melted by heat. Thus, thesemethods have been found not simple. Particularly, where powder materialssuch as sodium salt, potassium salt, barium salt and lead salt areemployed, it is not so easy to mix the starting materials homogeneously.In addition, it is necessary to melt the mixed starting materials, forexample, in a crucible for a long period of time. As a result,impurities tend to be introduced into the starting materials during thismelting operation. The resultant multi-component glass fiber preform hasoften failed to provide for optical fibers with low loss.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a method forproducing a multi-component glass fiber preform which method is quitesimple and ensures that a multi-component optical fiber with low loss isprovided.

Another object is to provide an apparatus for producing such a preformwhich apparatus is simple in construction.

According to a first aspect of the invention, there is provided a methodfor producing a multi-component glass fiber perform which comprises thesteps of nebulizing an aqueous solution of at least one metal salt, andreacting the atomized solution and a gaseous glass raw material withoxygen gas at a high temperature to produce particulate glass materialdeposited on a substrate.

The glass raw material is SiCl₄, and if desired, one or more of GeCl₄,POCl₃ and BBr₃ may be added. The aqueous solution is prepared using atleast one metal salt selected from the group consisting of alkali metalnitrate, alkali metal carbonate, alkali metal sulfate, alkali metalacetate, alkaline earth metal nitrate, alkaline earth metal carbonate,alkaline earth metal sulfate, alkaline earth metal acetate, leadnitrate, lead carbonate, lead sulfate, lead acetate, lanthanium nitrate,lanthanium carbonate, lanthanium sulfate and lanthanium acetate. Theglass raw material and the nebulized solution are mixed together andreacted with oxygen gas at a high temperature to produce particulateglass material or soot deposited on the substrate to form amulti-component glass fiber preform. The aqueous solution of metal saltor salts is nebulized using a nebulizer utilizing supersonic vibrationor an nebulizer utilizing gas under pressure. The substrate may be inthe form of a bar. In this case, the particulate glass material may bedeposited on one end or the outer periphery of the substrate bar ofcircular cross-section. The substrate may take the form of a tube, inwhich instance the particulate glass material is deposited on the innerperiphery of the substrate tube. During the deposition operation, thebar is axially rotated and moved, and the tube is axially rotated. Thepreform is drawn axially to form an optical fiber.

The amount of the dopant, i.e., the metal salt or salts contained in thepreform can be increased so that the refractive index of the preform canbe controlled to form an optical fiber either of the core clad type orthe graded index type.

The substrate tube may be made of silica. The particulate glass materialis deposited on the inner periphery of the tube to produce amulti-component glass fiber preform. The tube was heated to collapse itshollow portion to provide a solid construction. The preform is drawnaxially together with the tube to provide a jacketed optical fiber, thetube constituting the jacket for the optical fiber. The jacketed opticalfiber has an increased mechanical strength.

According to a second aspect of the invention, there is provided anapparatus for producing a multi-component glass fiber preform whichapparatus comprises a multi-conduit burner having five concentricconduits, the centrally disposed first conduit and the second and thirdconduits adjacent thereto being flush with one another at their tipends, the fourth conduit interposed between the third and outermostconduits extending axially beyond them, the first to fifth conduitsserving to feed a gaseous glass raw material, a fuel gas, an nebulizedaqueous solution of at least one metal salt, an inert gas and oxygengas, respectively, the burner having a nozzle adapted to be directed toan axially rotating and moving substrate to deposit particulate glassraw material on the substrate. With this apparatus, the dopant contentof the glass fiber preform can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a preform forming apparatus of thisinvention;

FIG. 2 is a view similar to FIG. 1 but showing a modified apparatus;

FIG. 3 is a schematic view of a multi-conduit burner;

FIG. 4 is a view similar to FIG. 1 but showing another modifiedapparatus;

FIG. 5 is a view similar to FIG. 1 but showing a further modifiedapparatus; and

FIGS. 6 to 9 are graphs showing the refractive index profile in thedirection of diameter of optical fiber preforms provided in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As the gaseous glass raw material, silicon chloride SiCl₄ gas is used.One or more of germanium chloride gas GeCl₄, phosphoryl chloride gasPOCl₃ and boron bromide BBr₃ may be added to SiCl₄ to prepare the glassraw material. SiCl₄, GeCl₄, POCl₃ and BBr₃ are first purified by precisedistillation, and then are gasified. The gasification is carried out byslowly heating a container filled with these materials and bubbling acarrier gas through the materials in the container. High purity argongas, helium gas or oxygen gas can be used as the carrier gas.

The dopant is at least one metal salt selected from the group consistingof alkali metal nitrate, alkali metal carbonate, alkali metal sulfate,alkali metal acetate, alkaline earth metal nitrate, alkaline earth metalcarbonate, alkaline earth metal sulfate, alkaline earth metal acetate,lead nitrate, lead carbonate, lead sulfate, lead acetate, lanthaniumnitrate, lanthanium carbonate, lanthanium sulfate and lanthaniumacetate. As the alkali metal, lithium, sodium, potassium and cesium canbe used. As the alkaline earth metal, magnesium, calcium and barium canbe used. The aqueous solution of metal salt or salts is subjected toextremely high purification, using a solvent extraction method or an ionexchange resin. The purified solution is nebulized by a nebulizerutilizing supersonic vibration or a nebulizer utilizing a carrier gasunder pressure. Preferably, an inert gas such as argon and helium gasesof high purity is used as the carrier gas.

Oxygen gas serving to oxidize the glass raw materials and the nebulizedsolution should preferably have a purity of more than 99.99%.

The invention will now be described with reference to the drawings.

FIG. 1 shows a preform forming apparatus 10 which comprises amulti-conduit burner 11 having a plurality of concentric conduits. Agaseous glass raw material, H₂, O₂ and a carrier gas such as argon orhelium are fed simultaneously through their respective conduits. Thehydrogen gas is burnt to form a flame 12. As described above, the glassraw material is composed of SiCl₄ and one or more of GeCl₄, BBr₃ andPOCl₃. The glass raw material is gasified by the bubbling operation asdescribed above. An aqueous solution of metal salt or salts, which serveas a dopant as described above, is highly purified and charged into anebulizer 13 utilizing supersonic vibration. The aqueous solution isnebulized and injected from a nozzle 14 into the flame 12. In the flame12, the glass raw material and the nebulized material is subjected bythe oxygen gas to chemical reactions such as flame hydrolysis andoxidation to produce a multi-component particulate glass material orsoot. The particulate material is directed by the flame 12 toward anddeposited on the lower end of a substrate bar 15. The particulatematerial or soot comprises oxides of the glass raw material such asSiO₂, GeO₂, B₂ O₃, and P₂ O₅, oxides of the metal salt such as Na₂ O,MgO and PbO, and oxides such as those containing Si-O-Na bond, Si-O-Mgbond and Si-O-Pb bond. The substrate bar 15 is supported on a lathe (notshown) for axial rotation and upward movement along the axis. Themulti-component particulate glass material or soot is deposited on thelower end of the thus rotating and moving bar 15 to form a porousmulti-component glass fiber preform 16. During the deposition operation,by changing the flow rates of the glass raw material and the nebulizedmaterial, the distance between the nozzle of the burner 11 and the lowerend of the substrate bar 15, and the angle of the burner 11 with respectto the substrate 15, the glass fiber preform 16 having a desiredrefractive index profile can be obtained.

FIG. 2 shows a modified preform forming apparatus 20. A gaseous glassraw material, oxygen gas, hydrogen gas and a carrier gas such as argonor helium gas are simultaneously fed through respective conduits of amulti-conduit burner 21, as described above for the burner 11. Asdescribed above, the glass raw material is composed of SiCl₄ and one ormore of GeCl₄, POCl₃ and BBr₃. An aqueous solution of metal salt orsalts, purified according to the procedure described above, is containedin a nebulizer 23. Argon gas under pressure is supplied as a carrier gasinto the nebulizer 23 through a pipe 24a so that the purified solutionis nebulized and injected from the nozzle 24 into a flame 22 of theburner 21. The glass raw material and the nebulized material areoxidized by the oxygen gas to allow their oxides in the form ofparticular glass material or soot to deposit on the outer periphery of asubstrate bar 25 to produce a porous multi-component glass fiber preform26. The substrate bar 25 is supported on a lathe (not shown) for axiallyrotation and reciprocal movement along the axis during the depositionoperation. By controlling the feed rate of one or more components of theglass raw material and nebulized metal salt solution during thereciprocal movement of the substrate bar 25, the refractive index of thepreform 26 can be changed either in a continuous or a stepped manner inthe radial direction of the substrate bar 25.

FIG. 3 shows a multi-conduit burner 31 having five concentric conduits31a to 31e. The first to fifth conduits 31a to 31e serve to feed theabove-mentioned glass raw material, hydrogen gas, the above-mentionednebulized metal salt solution, argon gas and oxygen gas, respectively.The gaseous glass raw material is fed through the centrally disposedfirst conduit 31a by a carrier gas such as argon gas. The first to thirdconduits 31a to 31c are flush with one another at their tip ends. Thefourth conduit 31d is about 15 to 20 mm longer than the first to thirdconduits and also is about 5 mm longer than the fifth conduit 31e. Withthis construction, a mixture area 32 is defined by the fourth conduit31d and the tip ends of the first to third conduits 31a to 31c. Themixture area 32 serves to sufficiently mix the materials emitted fromthe first to fourth conduits 31a to 31d. A flange member 33 is mountedaround the outermost or fifth conduit 31e and extends axially beyond thefourth conduit 31d. Hydrogen gas is burnt to form a flame 34.

The glass raw material and the nebulized metal salt solution, emittedrespectively from the conduits 31a and 31c, are adequately mixedtogether and directed into the flame 34 so that the particulate glassraw material or soot is produced by oxidization and deposited on thelower end of a substrate bar 35 to produce a porous multi-componentglass fiber preform 36, as shown in FIG. 4 which schematicallyillustrates a further modified preform forming apparatus 30. Thesubstrate bar 35 is supported on a lathe (not shown) for axial rotationand movement along the axis during the deposition operation. With thismethod, the dopant content of the obtained preform can be easilyincreased to a desired level. The substrate bar 35 may be arranged forhorizontal reciprocal movement so that the glass fiber preform is formedaround the outer periphery of the substrate. Further, as describedabove, by controlling the feed rate of one or more components of thematerials supplied through the multi-conduits burner 31, the refractiveindex of the preform can be controlled. Also, as described above, bychanging the flow rates of the glass raw materials and nebulizedmaterials, the distance between the nozzle of the burner 31 and thelower end of the substrate bar 35, and the angle of the burner 31 withrespect to the substrate 35, the glass fiber preform having a desiredrefractive index profile can be obtained.

FIG. 5 shows a further modified preform forming apparatus 40 whichemploys a process commonly known in the trade as "modified chemicalvapor deposition process". An elongated hollow substrate 41 in the formof silica tube is used. The substrate tube 41 is supported by spacedsupport portions 42a, 42a of a lathe 42 for axial rotation. A burner 43using H₂ and O₂ is mounted on the lathe 42 beneath the substrate tube 41for reciprocal movement therealong to heat the rotating tube 41. Thetube 41 has a smooth bore or inner peripheral wall which is cleaned. Theabove-mentioned gaseous glass raw material and nebulized solution,oxygen gas and the carrier gas are introduced into the bore of therotating tube 41. The glass raw material and nebulized material areheated by the moving burner 43 so that the particulate glass material orsoot is produced through chemical reactions such as flame hydrolysis andoxidization and is deposited on the inner peripheral surface of thesilica tube 41 to form a porous multi-component preform 44. As describedabove, by controlling the feed rate of one or more components of theglass raw material and nebulized metal salt solution during thereciprocal movement of the burner 43, the refractive index of thepreform 44 can be changed either in a continuous or a stepped manner.

For fabricating a multi-component glass fiber from the multi-componentglass fiber preforms produced according to the procedures shown in FIGS.1 and 4, the preform is heated to a temperature above a melting pointfor vitrification to provide a transparent glass preform. The vitrifiedpreform is then drawn to form an optical fiber. More specifically, wherea core preform and a cladding preform are formed on separate substratebars, the two preforms are introduced respectively into concentricallydisposed inner and outer chambers of a crucible, e.g., a platinumcrucible and are melted. The preforms thus treated are drawn to providea mutli-component optical glass fiber having a core portion and acladding portion. Alternatively, a core preform is formed on the lowerend of the substrate bar, and thereafter a cladding preform is formed onthe core preform to provide an integral preform. This integral preformis heated for vitrification to obtain a transparent preform. Thistransparent preform is drawn to provide a multi-component optical glassfiber.

For fabricating a glass fiber from the preform prepared according to theprocedure shown in FIG. 2, the preform is vitrified to obtain atransparent glass preform in the manner described above. Then, thesubstrate bar is removed from the preform, and this hollow preform isheated to collapse the hollow portion to provide a solid construction.In the case of the preform of which refractive index is varied in theradial direction, the preform is simply drawn to form a multi-componentoptical glass fiber. In the case of the preform of which refractiveindex is constant in the radial direction, the preform is formed into arod of a predetermined diameter. Then, a cladding is applied over thepreform, and the preform with the cladding is drawn to provide amulti-component glass fiber.

For fabricating a glass fiber from the preform prepared according to theprocedure shown in FIG. 4, the substrate tube, in which the preform isformed, is heated to collapse the hollow portion to form a solidstructure. The preform with the silica tube is drawn axially from oneend to form a multi-component optical glass fiber. The drawn tube 41serves as a jacket for the optical fiber and increases the mechanicalstrength of the fiber.

The invention will now be illustrated by the following examples:

EXAMPLE 1

An aqueous solution of 30% by weight NaNO₃ of extremely high purity wasprepared, using an ion exchange resin. The purified aqueous solution wascharged into a nebulizer 13 of a preform forming apparatus 10 shown inFIG. 1, the nebulizer 13 utilizing supersonic vibration. SiCl₄, GeCl₄,BBr₃ and POCl₃ serving as gaseous glass raw materials were fed through afirst conduit of a multi-conduit burner 11 at the flow rates of 200 ccper minute, 100 cc per minute, 50 cc per minute and 20 cc per minute,respectively, the multi-conduit burner 11 having a plurality ofconcentrically disposed conduits. Hydrogen gas serving as a fuel gas andoxygen gas were also fed separately through a second and a third conduitat the flow rates of 4000 cc per minute and 6000 cc per minute,respectively, the hydrogen gas being burnt to form a flame 12. Thepurified aqueous solution was subjected to supersonic vibration of 80kHz by the nebulizer 13, having a power of 50W, to be nebulized at thenozzle 14 of the nebulizer 13 and injected into the flame 12 so that theglass raw materials and the nebulized material were oxidized by theoxygen gas to allow their oxides in soot form to deposite on the lowerend of an axially rotating and moving substrate bar 15 to produce aporous multi-component glass fiber preform 16 for a core. The aqueoussolution to be nebulized was fed at a rate of 50 cc per minute.

Then, according to the procedure described above, a porousmulti-component glass fiber preform for a cladding was also prepared,using SiCl₄ (flow rate: 200 cc per minute), GeCl₄ (50 cc per minute),BBr₃ (50 cc per minute), and POCl₃ (20 cc per minute) serving as glassraw materials, hydrogen gas (4000 cc per minute), oxygen gas (6000 ccper minute) and an aqueous solution of 30% by weight NaNO₃ nebulized atthe rate of 40 cc per minute.

The thus obtained core preform and cladding preform were heated in afurnace, respectively, for vitrification to obtain two transparent glassperforms. The vitrified core preform and cladding preform wereintroduced respectively into concentrically-disposed inner and outerchambers of a platinum crucible. Then, the two preforms were drawn atthe rate of 20 cm per minute at a temperature of 800° to fabricate amulti-component optical glass fiber having a core portion and a claddingportion. The thus obtained optical fiber exhibited a refractive indexprofile of the step type as indicated by a graph in FIG. 6.

EXAMPLE 2

A multi-component optical glass fiber was obtained according to theprocedure of Example 1, except that an aqueous solution of 32% by weightKNO₃ was used instead of the aqueous solution of NaNO₃. The thusobtained optical fiber, like the optical fiber of Example 1, exhibited arefractive index profile of the step type.

EXAMPLE 3

An aqueous solution of 30% by weight MgSO₄ of extremely high purity andan aqueous solution of 30% by weight C_(S) SO₄ of extremely high puritywere prepared, using an ion exchange resin. The two solutions were mixedin equal amounts, and the resultant mixture solution was charged into anebulizer 23 of a preform forming apparatus 20 shown in FIG. 2. Argongas under pressure serving as a carrier gas was fed through a conduit24a to nebulize the solution.

SiCl₄ and POCl₃ serving as gaseous glass raw materials, H₂ and O₂ werefed through respective conduits of a multi-conduit burner 21, asdescribed above for the burner 11, at the flow rates of 100 cc perminute, 50 cc per minute, 2200 cc per minute and 3000 cc per minute. Thehydrogen gas was burnt to form a flame 22. The mixture solution wasnebulized at the nozzle 24 of the nebulizer 23 at a rate of 0.3 g perminute and injected into the flame 22 so that the glass raw materialsand the nebulized material were oxidized by the oxygen gas to allowtheir oxides in soot form to deposit on the outer peripheral surface ofa substrate bar 25, rotating about its axis and reciprocally movingalong its axis, to produce a porous multi-component glass fiber preform26. The substrate bar 25 was rotated at a rate of 20 r.p.m. andreciprocally moved at a speed of 300 mm per minute. During thedeposition operation, in order to continuously change the refractiveindex of the preform in its radial direction to obtain an optical fiberof the graded index type, the amount of SiCl₄ fed to the burner 21 wasincreased by 5 cc per minute in the range of between 100 cc per minuteand 200 cc per minute every one reciprocal movement of the substrate 10.

The thus obtained preform 26 was heated for vitrification in a carbonresistor furnace at a temperature of about 1400° C. to obtain atransparent glass preform. Then, the substrate bar 25 is removed fromthe preform, and the resulting hollow preform was heated to collapse thehollow portion to provide a solid construction. The solid preform wasdrawn from one end thereof to produce a multi-component optical glassfiber of the graded index type. The refractive index profile of theoptical fiber was matched substantially to a parabolic curve as plottedin FIG. 7.

EXAMPLE 4

A multi-conduit burner 31 (FIG. 3) having five concentric conduits 31ato 3e was employed. SiCl₄, POCl₃ and Ar were fed through the first orcentral conduit 31a at flow rates of 120 cc per minute, 10 cc per minuteand 50 cc per minute, respectively, SiCl₄ and POCl₃ constituting glassraw materials. H₂ was supplied through the second conduit 31b at a flowrate of 3 l per minute. Pb(NO₃)₂ and H₂ O mixed in the weight ratio of3:10 was fed through the third conduit 31c at a rate of 0.28 g perminute, the mixture being in nebulized form. Ar was fed through thefourth conduit 31d at a rate of 4 l per minute. O₂ was fed through thefifth conduit 31e at a rate of 3 l per minute. H₂ was burnt to form aflame 34 and the glass raw material and the nebulized mixture wereoxidized by the oxygen gas to allow their particulate oxides to depositon the lower end of a substrate bar 35 to produce a multi-componentglass fiber preform 36 (FIG. 4). The substrate bar 35 was rotatedaxially at a rate of 20 r.p.m. and moved upwardly at a speed of 40 mmper hour. The thus produced preform was passed through a carbon resisterfurnace, maintained at a temperature of about 1,300° C., forvitrification to obtain a transparent glass preform. The transparentpreform was drawn by a lathe to have a predetermined diameter orcross-section, and a tube of silica was fitted over the thus drawnmaterial. Then, the drawn material with the silica tube was heated at atemperature of about 2,000° C. to obtain a multi-component optical glassfiber. The refractive index profile of the optical fiber is shown inFIG. 8.

EXAMPLE 5

A multi-component optical glass fiber of the graded index type wasprepared, using a preform forming apparatus 40 shown in FIG. 5. Anaqueous solution of 30% by weight Pb(NO₃)₂ of high purity and an aqueoussolution of 20% by weight Ba(CH₃ COO)₂ of high purity were prepared,using either a solvent extraction method or an ion exchange resin. Thetwo solutions were mixed in equal amounts, and the resultant mixturesolution was nebulized by an nebulizer (not shown) using argon as acarrier gas and was introduced into a tube 41 of silica. The amount ofthe argon gas supplied was increased by 8 cc per minute in the range ofbetween 0 cc per minute and 400 cc per minute every one reciprocalmovement of a burner 43 along the tube 41, so that the amount of thenebulized mixture supplied was controlled to the range of between 0 gper minute and 0.5 g per minute.

Simultaneously with the supply of the nebulized mixture into the tube41, SiCl₄ and POCl₃, serving as glass raw materials, and O₂ were fedinto the tube 41 at flow rates of 150 cc per minute, 30 cc per minuteand 200 cc per minute, respectively. The tube 41 was supported by alathe 42 and axially rotated at a rate of 20 r.p.m. The tube 41 had aninner diameter of 18 mm, and the length of the tube between opposedsupport portions 42a, 42a was 1 m. The tube 41 was heated by the burner43 using H₂ and O₂ which burner was reciprocally moved along the tube 41at a speed of 30 cm per minute. The glass raw material and the nebulizedmaterial in the tube 41 were oxidized by the oxygen gas in the tube toallow their particulate oxides to deposite on the inner periphery of thetube 41 to produce a multi-component glass fiber preform 44. After theburner 43 reciprocally moved along the tube 41 fifty times, theintroduction of the materials into the tube 51 was stopped. Then, thetube 41 was heated to 1,900° C. to collapse the hollow portion of thetube to provide a solid structure. The solid preform with the tube wasdrawn at 2,100° C. at a speed of 30 m per minute to produce a jacketedmulti-component optical glass fiber. Thus, the tube 41 is simultaneouslydrawn to form jacket for the optical fiber. The refractive index profileof the optical fiber was matched substantially to a parabolic curve asplotted in FIG. 9.

What is claimed is:
 1. Apparatus for producing a multi-component glassfiber preform, which comprises a multi-conduit burner having fiveconcentric conduits, the centrally disposed first conduit and the secondand third conduits adjacent thereto being flush with one another attheir tip ends, the fourth conduit interposed between the third andoutermost conduits extending axially beyond them, the first to fifthconduits serving to feed a gaseous glass raw material, a fuel gas, anebulized aqueous solution of at least one metal salt, an inert gas andoxygen gas, respectively, and said burner having a nozzle adapted to bedirected to a substrate.